The broad goals of this proposal are to identify examples of the influence of individual HLA alleles as well as non-HLA, non-TCR genes on the TCR repertoire of both single-positive and double-negative cells, and to determine the extent to which these genes can alter the repertoire either by "positive" or "negative" selection. Such influences on the human TCR repertoire may have profound implications in terms of determining an individual's ability to respond to pathogens and/or development of an autoimmune disease. We propose to perform these studies on a highly inbred population. the Amish, using V-segment-specific monoclonal antibodies and qPCR as well as other methods that are being developed. Specifically, we propose to: 1) analyze the Valpha and Vbeta TCR repertoire in double-negative (DN) cells in identical twins and HLA-identical and non-identical sibs. This will help us in determining whether such T cells can express "forbidden" TCR which may be potentially autoreactive. 2) determine the influence of HLA and non-HLA genes on the Valpha TCR repertoire of single positive (CD4+ and CD8+) cells. 3) determine the molecular basis for the association of increased numbers of T cells expressing Valpha2.3 and the HLA specificity B38. 4) identify examples of "positive" and "negative" selection by HLA genes in the Amish: by-first, identifying additional examples of HLA associations with TCR and then, determining-the extent to which the TCR repertoire has been altered by the presence of that HLA allele. 5) identify examples in the Amish where TCR are genetically determined by non-HLA, nonTCR genes which may represent endogenous "superantigens". 6) continue the development of new methods for analyzing the TCR repertoire which will make it possible to more easily define the effects of individual HLA alleles or endogenous "superantigens" on the TCR repertoire.